1. Field of the Invention
The present invention relates generally to an improved interbody (for placement at least in part between adjacent vertebral bodies in the space previously occupied by disc material) spinal fusion implant for the immobilization of vertebrae. In particular, the invention relates to a spinal fusion implant that is selectively directionally expandable and which specifically has height raising capabilities that are utilized once the implant is initially positioned. Such height raising capability may be utilized within the spine anteriorly, posteriorly, or both and to various extents, respectively so as to raise the front, back, or both of the implant by the same or various amounts. More particularly, the invention relates to an implant having portions of upper and lower members that have a first, collapsed position relative to one another during insertion and a second, expanded position relative to one another allowing for an increased height. Further, the invention relates to cooperatively configured interlocking side walls of the upper and lower members that are adapted to hold the implant in an expanded position when moved from the collapsed position.
2. Description of the Related Art
Threaded and push-in spinal fusion implants having upper and lower portions adapted for placement in contact with adjacent vertebral bodies are known in the related art. The first artificial threaded spinal fusion implant was invented by Michelson and is disclosed in U.S. Pat. No. 5,015,247, filed Jun. 13, 1988, which is hereby incorporated by reference. Various push-in spinal fusion implants have been invented by Michelson and are disclosed in U.S. Pat. No. 5,593,409, filed Feb. 17, 1995 and U.S. Pat. No. 5,776,199, filed Jun. 28, 1988, which are hereby incorporated by reference.
Lordotic, frusto-conical, or tapered, threaded and push-in spinal fusion implants are also known in the art. By way of example, Michelson has invented such implants as disclosed in U.S. application Ser. No. 08/480,904 and EP 96917996.9, and U.S. Pat. No. 5,609,635, filed Jun. 7, 1995, which are hereby incorporated by reference.
Expandable fusion implants are known in the related art. The first expandable spinal fusion (allowing for the growth of bone from vertebral body to vertebral body through the implant) implant was invented by Michelson and is disclosed in U.S. Pat. No. 5,776,199, filed Jun. 28, 1988, previously incorporated by reference herein.
Lordotic, frusto-conical, or tapered, spinal fusion implants have the advantage of restoring or enhancing spinal lordosis. Threaded and push-in spinal fusion implants offer the advantage of being easily positioned in the implantation space and of having excellent fastening or holding features. Expandable fusion implants offer the advantage of allowing for the placement of a potentially larger implant through a smaller opening in a patient""s body. Selective expansion along a single direction, (e.g. vertically only when correctly installed) offers the advantage of increasing the height of the implant and therefore the distraction of the disc space, but without a concomitant increase in the width of the implant.
There exists a need for an artificial interbody spinal fusion implant providing for all of the aforementioned advantages in combination.
In accordance with the present invention, as embodied and broadly described herein, there is provided an expandable artificial interbody spinal fusion implant for insertion across a disc space between two adjacent vertebral bodies of a human spine. The implant of the present invention includes an upper member having a portion adapted for placement toward and into contact with or at least in part within one of the adjacent vertebral bodies and a lower member having a portion adapted for placement toward and into contact with or at least in part within the other of the adjacent vertebral bodies. The portions of the upper and lower members have at least one opening in communication with one another for permitting for the growth of bone from a vertebral body to an adjacent vertebral body through the implant. The upper and lower members are articulated therebetween, preferably proximate one of the proximal ends and the distal ends of the upper and lower members and preferably allow for divergence between the articulating members at the end opposite the articulating end of the implant. The upper and lower members have a first position relative to one another that allows for a collapsed implant height and a second position relative to one another that allows for an increased height. The portions of the upper and lower members in the first position of the present invention may be parallel or angled to one another. Preferably, at least a portion of a bone-engaging projection, such as a helical thread, ratchet, or knurling, is on the exterior of each of the opposed portions of the upper and lower members for engaging the adjacent vertebral bodies. The upper and lower members have a leading or distal end, an opposite trailing or proximal end, and a length therebetween. A cooperatively configured interlocking side wall of the upper and lower members is adapted to hold at least a portion of the upper and lower members apart so as to maintain the increased height of the implant and resist the collapse of the implant to the collapsed implant height. Expansion of the implant preferably increases the implant height only, that is in a plane preferably passing through the mid-longitudinal axis of the implant and the upper and lower members.
Each of the upper and lower members of at least one embodiment of the present invention have side walls adapted to cooperatively engage one another along at least a portion of the length of the side walls to hold at least a portion of the upper and lower members apart so as to maintain the increased height of the implant and resist the collapse of the implant to the collapsed implant height when the implant is in a final deployed position. Preferably the side walls of at least one, and if desired both, of the upper and lower members flex when the implant is moved from the first position to the second position. In particular, when the implant is moved from the first position to the second position certain of the side walls may spring from a position closer to the mid-longitudinal axis of the implant to a position further away from the mid-longitudinal axis or may spring from a position further from the mid-longitudinal axis of the implant to a position closer to the mid-longitudinal axis. This movement of at least one of the side walls may include rotating at least a portion of the side wall along an arc around an axis that is parallel to the mid-longitudinal axis of the implant when the implant is moved from the first position to the second position. These rotational, flexing, or springing forces acting to engage the cooperatively configured side walls of the upper and lower members together add lateral stability to the implant while maintaining the increased height of the implant.
Preferably, the side walls of the upper and lower members have cooperatively engaging stepped surfaces. Preferred stepped surfaces of one embodiment of the present invention include interdigitating projections and detents. The side walls having detents preferably have more detents than the cooperatively engaging side walls having projections have projections to permit the implant to move from the first position to the second position. During movement of the implant from the first position to the second position the side walls having detents in one embodiment have at least one detent that narrows during movement of the side wall having detents. This movement moves the side wall having detents into alignment and engagement with the projections of the wall having projections to increase the height of the implant. One particular preferred embodiment of the present invention includes side walls with two projections adapted to cooperatively engage side walls with three detents. Moving the two projections from the lower two of the three indentations to the higher two of the three indentations results in an increase to the maximum height of the implant.
The implant in one embodiment may be expanded with an extrinsic tool and then the expanded portions held apart in the second position by the interlocking side walls of the upper and lower members. The present invention includes expanding the implant with a tool, such as a spreader or a distractor, but is not limited to a scissors type, a rack and gear type, a threaded member type or any other type of particular external expander tool mechanism. Each tool nevertheless preferably engages the upper and the lower implant members to urge the implant apart. Then the interlocking side walls of the upper and lower members maintain the implant at an expanded height. The amount of the increase in the height of the implant may vary depending upon the amount of distraction of the implant desired by the physician.
Preferred forms of interbody spinal fusion implants have a substantial hollow portion. Certain expandable interbody spinal fusion implants that increase in height only of the related art contain an expansion mechanism passing longitudinally therethrough or an expansion mechanism that is configured for movement of the expansion mechanism from proximate one end of the hollow portion to proximate the other end of the hollow portion, thus requiring the expander to pass through the length of the hollow portion. A preferred embodiment of the present invention overcomes these limitations.
The portions of the upper and lower members are moved from a parallel orientation to an angled orientation; or a parallel orientation to an increased height parallel orientation; or an angled orientation to a parallel orientation; or an angled orientation to an increased height angled orientation that may be the same or a different angle relative to one another; or from a first height at each end to a second and greater height at at least one and possibly both ends. Each of the upper and lower members structurally cooperates with one another via the interlocking side walls so as to keep the implant in its expanded position.
The implant is preferably packed full of bone or other fusion-promoting substances prior to expansion of the implant. Expansion of the implant results in a space being formed in the implant interior into which additional fusion promoting substances such as bone may preferably be packed.
When installing a preferred implant from the posterior approach to the spine, the implant is driven from the trailing end and the leading end at the anterior aspect of the spine is raised or expanded. When expanded, the implant installed from the posterior aspect leaves a void at the leading end of the implant near the anterior aspect of the spine because the leading end of the implant has been made taller, the void preferably being packed with bone after expansion of the implant. Additionally, any path left behind in the bone filled interior of the implant by any tool passing through the bone filled interior is preferably packed with bone as well.
The implant may have an overlapping step-cut wall junction between the upper and lower members, which offers as some of its advantages: increasing the lateral rigidity of the implant, holding the implant in the closed first position until expanded, and to the extent desired retaining the fusion-promoting materials within the implant. The wall junction may be either solid or perforated.
One of the upper and lower members preferably has an interior wall extending toward the other of the upper and lower members and, more preferably, has two interior walls extending from each side thereof. The interior walls may be aligned parallel with the longitudinal axis of the implant. The other one of the upper and lower members preferably has an interior-contacting surface adapted to contact or receive the interior longitudinal wall.
By way of example, one of the upper and lower members may have a longitudinally extending interior wall, which is preferably unexposed, extending toward the other of the upper and lower members when the implant is in an initial insertion position. When the implant is in the final expanded or deployed position the implant has a preferred shape such that each of the portions of the upper and lower members are separated by at least a portion of interior wall, which in this position preferably has an exposed side.
The upper and lower members in certain embodiments are articulated to one another so one of the respective ends of the upper and lower members remain articulated while the other of the respective ends of the upper and lower members are free to move away from one another. In a preferred embodiment, the articulating means is achieved without a third member, such as an axle shaft, for example, passing through the implant. The articulating means preferably is formed into the implant walls themselves, and in a further preference in such a way that the two-implant halves may be articulated when at 90 degrees to each other. The halves then are moved, much like a book closing, toward each other prior to insertion into the implantation space in the spine. Once the upper and lower members are closed from the approximately 90 degrees articulating position, much like closing the leaves of a book, the upper and lower members of the implant are locked together at the articulation so that the members will not disarticulate when in use. Other types of articulation as would be known to one of ordinary skill in the art are within the scope of the present invention.
By way of example, the upper and lower members preferably have a cooperating rotational articulation or pivot point between a proximate one of the proximal end and the distal end of the upper and lower members. The cooperating rotational articulation preferably is proximate one of the proximal end and the distal end of the upper and lower members at an end opposite to the end to be expanded. A preferred rotational articulation configuration includes cooperating brackets and projections configured such that articulation therebetween occurs when the upper and lower members are substantially perpendicular to one another. Such a configuration offers the advantage that the brackets and the projections will not disengage one another when articulated for use such as insertion into the spine and subsequent expansion within a range of movement of the upper and lower members resulting from expanding the implant.
When the implant is in the final or expanded position the implant in one of the referred embodiments may take the general form of a cylinder or frusto-conical shape split along a horizontal plane through its mid-longitudinal axis wedged upper half from lower half by an inclined plane.
At least one and preferably both of the upper and lower members may have a screw hole passing through the trailing end, which preferably is adapted to receive a screw passing through the end of the upper and lower members and from the interior of the implant into each of the adjacent vertebral bodies to anchor the implant, further stabilize those vertebral bodies relative to each other, prevent undesirable motion at the vertebral body implant interfaces, increase the compressive load at the implant trailing end, prevent rocking; and thus mitigate against excessive peak loads and more uniformly distribute loads imparted to the implant over the length of the implant to the adjacent vertebral bodies. The implant may have a side configured, when in the expanded position, to cooperate with another interbody spinal fusion implant so as to allow the pair of implants to have a reduced combined width.
The trailing end of the implant preferably has a tool-engaging portion, but the implant may be adapted to cooperatively engage a driver at another location or by any means as would be known to one of ordinary skill in the art. This tool-engaging portion is adapted to engage an insertion tool that holds the implant during insertion in the spine. The configuration of the tool-engaging portion may be an opening, and more particularly an opening that is along the longitudinal axis of the implant. It is appreciated that the tool-engaging portion need not be an opening. A hole or a blind hole, threaded or otherwise, is preferred in another embodiment. In another preferred embodiment the opening preferably is a threaded slot that functions to cooperatively engage and disengage a tool for use in inserting the implant. In specific embodiments, the leading or trailing end may have wall portions, and/or be adapted to cooperatively engage a cap. Either the end wall portions or a cap may have an opening or openings that may function to hold fusion-promoting materials within the implant and/or, permit vascular access and bone growth therethrough.
By way of example, an implant configured for insertion from an anterior approach may be initially packed from the distal or leading end of the implant. The implant is then driven into position. Once the implant is expanded into the final position and any associated tool for expanding the implant is withdrawn from the implant, any void in the bone packed into the implant interior may be filled.
The accompanying drawings, which are incorporated in and constitute a part of this specification, are by way of example only and not limitation, and illustrate several embodiments of the invention, which together with the description, serve to explain the principles of the invention. The scope of the invention is limited only by the scope of the claims as from the present teachings other embodiments of the present invention shall be apparent to those skilled in the art.